90 



• KNO\VLEDGE ♦ 



[July 



Tlu- liiiul of the comet wixs in sharp focus upon the slit, 

 aiul the coutiiiuous spectrum with <kiiiieil edges corresponds 

 to tlie nuch'us, which in this comet was very distinct In 

 this continuous spectrum at least live separate places of 

 greater l)rjghtness are seen, which very pi-obably represent 

 groups of bright lines, though they are not sulliciently 

 distinct in the photograph to admit of resolution. 'J'hat 

 this interpretation is correct, seems probable, from the 

 circumstance that these groups, as shown in the diagram, 

 project beyond the strong continuous spectrum on one 

 side. The side corresponds to where the light of the coma, 

 on the side of tlie nucleus ne.\t the sun, falls upon the 

 slit We learn, therefore, that the light of this part of 

 the coma consists for the most part, m this part of the 

 spectrum, of these groups, as here on the plate onlj- an 

 excti^dingly faint continuous spectrum can be seen. 



It is not possible to measure with any useful accuracy 

 tlie U^ginnings and endings of the groups, as they arc too 

 faint at these points. ^Measures as accurate as the circum- 

 stances -would permit liave been taken of the brightest 

 parts of the groups. The wave-lengths of these brightest 



A 4253 I 



,\ 4412 

 parts are : \ 4507 



,\ 4G34 



,\ 4769 J 

 In the visible spectrum the bright lines of sodium 

 appear to have been strong, and it may be that some of 

 the light of some of the groups may be due to this 

 substance. 



Professor A. Herschel and Dr. von Konkoly showed 

 long ago that the spectra of the periodic meteors are 

 difl'erent for different swarms, and it does not seem sur- 

 jirising that we have now a comet, the matter of the nucleus 

 of which under the sun's heat shows an essential chemical 

 difference from the long series of hydrocarbon comets which 

 have appeared since lb04. 



Mr. Hind has kindly furnished me with the distance of 

 this comet from the sun at the time the photograph was 

 taken. The comet was then 42,380,000 miles distant from 

 the sun, while the comet of last year was CO, 420,000 miles 

 [distant] \vhcn I obtained the photograph of its spectrum. 



AIDS TO THE STUDY OF GEOLOGY. 

 Bv W. Jdrome H.VRRI80X, F.G.S. 



AT the request of the Editor I have undertaken to con- 

 tribute a series of geological papers to Knowledge. 

 These papers will take the form of accounts of visits made 

 to places of geological interest, e.specially to such as are 

 well-known holiday resorts, and are easy of access. I 

 shall also give an account of the geology of some of our 

 grtrat towns, showing what may bo learnt by a study of 

 the stones used in thera for building, paving, Ac, and 

 what natural sections are exposed in their immediate 

 nciglibourhoods. 



In this preliminary paper I shall describe the apparatus, 

 Ixxiks, Ac, whicli are required for use in the field or in the 

 study, by those who desire to investigate the rocks. 



Appakatcs for Field-wokk. — A good 'nuip is indis- 

 pensable. The geologist should provide himself with both 

 the plain map (Ordnance Department, Quarter-Sheets, Is. 

 each) and the coloured map (Geological Survey, Quarter- 

 Sheets, .Is. f^ch). The former will give him the hills, 

 while the latter will show where the various beds of rock 

 occupy the surface. Each quarter-sheet should be mounted 



on linen, so as to fold into six ; it can then be conveniently 

 carried in the pocket. Catalogues of the maps and other 

 publications of these two tSovcrnmcnt Departments may 

 be obtained from Stanfoi-d's, Charing-cross, and from these 

 the particular maps rtMniired, according to the district to 

 be examined, can be selected. 



Another necessary article is a hammer : the head should 

 weigh about one-and-a-quarter pounds, and have a square 

 face, made of mild steel ; the handle is best made of ash, 

 and should be divided into inches, so as to serve as a 

 measure. The shape and weight of the hammer will vary 

 with the nature of the rocks on which it is to be used ; 

 for old tough rocks, like those of Wales, the head should 

 be short and heavy ; while for work among soft and easily 

 splitting beds a lighter and longer hamnier-hoad is de- 

 sirable. The wedge-end of the hammer is Ijcst made at 

 right-angles to the shaft When rock specimens an^ 

 desired, a light trimming-hammer should also be carried, 

 to reduce the specimens into shape in the Held. A cohl 

 chisel is a useful companion to the hammer ; it enables 

 fossils, crystals, Ac, to be detached, without the risk 

 which often attends their removal from the rock by 

 means of blows from a hammer. 



The hammer should be carried by means of a leather 

 belt, provided with a slit through which the handle may bo 

 passed, and a leather flap to keep the hammer-head from 

 soiling or chafing the clothes. To the belt a small leather 

 case, containing a cowpas.'', may also be attached. A 

 geologist cannot keep to the roads, and must have the 

 means, not only of steering his own path, but also of 

 ascertaining the direction in which the rocks run, or their 

 strike, as it is called ; and for each of these purposes a 

 compass is necessary. 



It is a rare thing in this country to see a bed of rock 

 lie horizontally for any great distance ; any railway cutting 

 or {juarry will usually show the strata slanting or dippimj 

 in some direction or ether ; the direction may be ascer- 

 tained by the compass, but to measure the degree, or anffh 

 of dip, we use an instrument termed a clinometer. By 

 hanging a little pendulum to the central point, marking 

 degrees on the circle, and adding a short, flat base, we can 

 make the compass serve also as a dip-measurer, or clino- 

 meter. This form of instrument, complete in leather case, 

 is usually sold at one guinea ; but a much cheaper clino- 

 meter can be made by drawing a graduated semicircle on a 

 thin, flat, oblong piece of boxwood, and pivoting a bi-ass 

 pendulum at the centre of the arc. 



The Wa/lel, or L'tir/, must not be forgotten, for an 

 enthusiastic g(!ologist seldom fails to meet with many 

 specimen.s, in the course of a day's outing, which he will 

 desire to take home for further examination, or to add to 

 his collection ; a leather game bag answers capitally when at 

 work among the igneous or slaty rocks, whoso weight and 

 sharp angles rapidly cut through anything less tough than 

 leather. The satchels made of a stout brown material 

 which are much used to carry school-books in, also make 

 capital collecting-bags. A good supply of brown paper 

 should be provided, so that each specimen may be wrapped 

 up separately, while, if fragile shells or delicate crystals are 

 likely to be obtained, some chip boxes and cottonwool 

 must also be taken. To identify specimens it is best to 

 carry gummed strips of numbers ; these should be detached, 

 one by one, and affixed to each rock or fossil at the time it 

 is obtained, an entry' being made at the same time in a 

 note-book of the locality and exact position whence the 

 specimen was derived, with any other particulars of 

 interest 



Many geologists always carry a little dilut(! hydrochloric 

 acid, in a small stout bottle, in order to be able to identify 



